This invention relates to neodymium-doped phosphate laser glasses having a high stimulated emission cross-section together with desirably low self-concentration quenching of the neodymium fluorescence and high thermal shock resistance, in comparison to prior-art and commercially-available phosphate laser glasses.
Glass is an attractive host material for neodymium lasers. It is easy to prepare in high optical quality and large-size relative to crystals, and a broad and continuously adjustable range of properties may be produced.
For high average power, high repetition rate glass lasers useful for materials processing, or other applications such as miniaturized lasers for integrated optics, it is desirable to provide active materials with high induced emission cross-sections (.sigma.) which can tolerate high dopant levels without significant self-concentration quenching, as the gain of the device is proportional to the product of .sigma. and the number of activated ions. Most solid-state laser materials such as YAG or glass exhibit significant self-concentration quenching (i.e., transfer of excited state energy between adjacent activator ions) at relatively low ion concentrations, typically.about.2.times.10.sup.20 cm.sup.-3 for glasses. At higher concentrations the quenching becomes increasingly severe, e.g., in the prior art, commercially-available phosphate glass LG-750.
The first material reported to have extremely low concentration quenching was the crystalline ultraphosphate NdP.sub.5 O.sub.14, (Danielmeyer, H. G.; Weber, H. P.; IEEE J. Quant. Electron. EQ-8, 805 (1972)) and its glassy analog (Damen. et al., U.S. Pat. No. 3,863,177, Jan. 28, 1975). While glassy NdP.sub.5 O.sub.14 has desirable laser properties, it is virtually impossible to produce optical quality glass in commercial quantities and sizes due to its crystallization tendency and the high level of phosphorus volatility, which necessitates production in sealed pressure bombs. Since its introduction, many low quenching rate crystals have been reported.
Low self-concentration quenching rate glasses described in the open literature appear to be confined to two main families, the first in the system R.sub.2 O.Ln.sub.2 O.sub.3.P.sub.2 O.sub.5 and the second in the system R.sub.2 O.Al.sub.2 O.sub.3.Nd.sub.2 O.sub.3.P.sub.2 O.sub.5 where R.sub.2 O is Li.sub.2 O, Na.sub.2 O, or K.sub.2 O and Ln=La+Nd. A summary of compositional effects on concentration quenching rate and laser properties in these systems has been given by Cook, et al., SPIE, Proc. 505, 102 (1984) (ref. 1). The composition with the lowest concentration quenching rate reported in that study was Composition 16. This composition was a simple alkali ultraphosphate. Although its quenching rate was somewhat higher than glassy NdP.sub.5 O.sub.14, its fluorescence lifetime for all neodymium concentrations below approximately 2.times.10.sup.21 ions/cm.sup.3 was substantially above the latter glass. Additionally, its emission cross-section was substantially higher than glassy NdP.sub.5 O.sub.14, making it of greater practical utility for high average power lasers. The glass was also sufficiently stable to allow production by conventional glassmaking techniques.
In the operation of a high average power, high repetition rate glass laser, the exposure of the glass to pump light coupled with absorption of the light by neodymium results in heat build-up within the glass. As laser efficiency decreases rapidly with temperature, it is common for such lasers to be cooled by liquids or gases. This leads to a temperature gradient, and thus a stress gradient, in the glass which can lead to fracture if stresses are great enough. This has been well discussed in the literature; for example, see H. Rawson, PROPERTIES AND APPLICATION OF GLASS, Elsevier 1980, pp. 82-86.
For the case of a sheet of glass of temperature (T.sub.i), which is then cooled to a lower surface temperature, T.sub.o, the surface tensile stress, S.sub.t, produced by the temperature drop is given by, EQU S.sub.t =E.alpha.(T.sub.i -T.sub.o)/(1-v) (1)
where E is Young's Modulus, .alpha. is the coefficient of thermal expansion, and .sigma. is Poisson's Ratio. Based on the need for a glass laser to survive the largest possible temperature gradient in operation, a Figure of Merit (FOM) formula derived from the above equation is often used to rank laser glasses for resistance to thermal shock under moderate heat flow conditions, EQU FOM=S.sub.t (1-v)*K/E.multidot..alpha. (2)
where K is the thermal conductivity, and S.sub.t is the tensile strength of the glass (S. W. Freiman, "Fracture Mechanics of Glass", pp. 21-78 in GLASS: SCIENCE AND TECHNOLOGY, D. Uhlmann, N. Kreidl, eds. Academic Press 1980).
The FOM is directly proportional to the maximum temperature gradient that the glass can withstand before fracture occurs; thus the higher the FOM, the more suitable a glass will be for the above applications. Because the range of tensile strength values for glass is primarily determined by the distribution of surface flaws rather than intrinsic material properties, the term S.sub.t is generally omitted from the FOM when making relative comparisons. FOM values quoted hereafter are so calculated.
Table 1 gives a comparison of thermal FOM and related physical and laser properties for the low quenching rate ultraphosphate no. 16 of ref. 1 and a number of commercially-produced phosphate laser glasses. It is clear that the ultraphosphate is unsuitable for practical applications because of its low FOM.
Again, neglecting tensile strength, the physical properties whose variation produce the largest improvement in FOM are the thermal expansion coefficient, Young's Modulus, and thermal conductivity. Thus, for the highest possible FOM one would desire a glass with the lowest possible .alpha. and E, and the highest possible K. The compositional influence on the aforementioned physical properties in phosphate glasses is generally known, as reviewed in N. H. Ray, "Compositional-Property Relationships in Inorganic Oxide Glasses," J. Non-Cryst. Solids 15 (1974), p. 423-34. In phosphate glasses, improvements in FOM, i.e., reduction in .alpha. and E with increase in K, have been most commonly achieved by additions of Al.sub.2 O.sub.3. Indeed, virtually every prior-art phosphate laser glass contains substantial amounts of aluminum oxide. However, as can be clearly seen in ref. 1, the addition of aluminum to ultraphosphates leads to strong increases in concentration quenching rates and decreases the emission cross-section at those concentrations which are effective in raising the thermal FOM. Thus, in contradiction to prior-art disclosures, the incorporation of Al.sub.2 O.sub.3 is contraindicated for laser glasses suitable for high average power applications.
TABLE 1 __________________________________________________________________________ Data is from Manufacturers' Catalogs Unless Otherwise Noted Ti at Ti at Ti at Manufacturer/ K .alpha. E .sigma. 0.5% (.mu.sec) 3% (.mu.sec) 10% (.mu.sec) Thermal Glass Code (W/m .multidot. K) .times. 10.sup.-7.degree. C.sup.-1 10.sup.3 N/mm.sup.2 .nu. (.times. 10.sup.-20 cm.sup.2) Nd.sub.2 O.sub.3 Nd.sub.2 O.sub.3 Nd.sub.2 O.sub.3 F.O.M. __________________________________________________________________________ Composition 16, .49 131 45 .270 4.0 381 .mu.sec 215 .mu.sec .61 ref. 1 Schott LG-750 .62 130 50.1 .256 4.0 390 .mu.sec 330 .mu.sec 160 .mu.sec .71 LG-760 .67 138 53.7 .267 4.3 380 .mu.sec 335 .mu.sec 150 .mu.sec .66 Hoya LHG-5 .77 98 67.8 .237 4.1 N.D. 290 .mu.sec N.D. .88 LHG-7 .sup. .72.sup.1 112 55.3 .238 3.8 N.D. 305 .mu.sec N.D. .88 LHG-8 .58 127 50.1 .258 4.2 410 .mu.sec 315 .mu.sec 200 .mu.sec .68 8% Kigre Q-88 .sup. .74.sup.1 92 69.9 .24 4.0 400 .mu.sec 330 .mu.sec 130 .mu.sec .87 Q-98 .82 99 70.7 .24 4.5 400 .mu.sec 350 .mu.sec 130 .mu.sec .89 Q-100 .82 115 70.1 .24 4.4 .77 __________________________________________________________________________ .sup.1 Source Laser Glass Handbook, M. J. Weber, ed., Lawrence Livermore National Laboratory
It is also known that both B.sub.2 O.sub.3 and SiO.sub.2 may be added to phosphate glasses in order to reduce the thermal expansion coefficient and improve the mechanical strength. This is generally discussed in LASER PHOSPHATE GLASSES, M. E. Zhabotinskii, ed., Nauka publ., Moscow, 1980. Phosphate laser glasses incorporating silica, and boron in theory, have also been disclosed very generically in the patent literature (see Japanese Pat. No. 7842334(no B), German Pat. No. DE 3435133 and U.S. Pat. No. 4,075,120). These glasses, particularly those disclosed in German Pat. No. DE 3435133, show thermal expansion coefficients of 80-94.times.10.sup.-7 C.degree. .sup.-1, which generally are significantly lower than the aluminophosphate laser glasses summarized in Table 2 below. Insufficient information was disclosed to allow calculation of thermal F.O.M. However, these silicaphosphate glasses do not have concentration quenching rates significantly different from aluminophosphate glasses (see Table 2, No. DE 3435133). The U.S. patent does not give any data on physical or laser properties of its examples; it incorporates SiO.sub.2 as an antisolarant only.